1. Field of the Invention
The present invention relates to a light control device using an electro-optic material whose refractive index varies as a result of the application of an electric field, and a light control system including the light control device.
2. Description of the Related Art
Recently, digital information recording systems utilizing the principle of holograms have been known as large-capacity recording systems. See, for instance, Patent Reference 1 (Japanese Patent Application Kokai No. 2002-297008).
For example, materials that have the electro-optic effect such as lanthanum lead titanate zirconate (hereafter referred to as “PLZT”) can be used as the material for a spatial light modulator of a hologram recording device. PLZT is a transparent ceramic having a composition of (Pb1-yLay) (Zr1-xTix) O3. The electro-optic effect refers to a phenomenon in which polarization occurs in a substance when an electric field is applied to the substance, so that the refractive index is varied. If the electro-optic effect is utilized, the phase of light can be switched by switching the application of a voltage on and off. Therefore, a light modulating material having the electro-optic effect can be applied to the light shutter of a spatial light modulator or the like.
In this type of application to elements such as a light shutter, bulk PLZT has been widely utilized. See, for example, Patent Reference 2 (Japanese Patent Application Kokai No. H5-257103). However, it is difficult for light shutters using bulk PLZT to meet the demand for miniaturization and integration and the demand for a reduction of an operating voltage and a cost reduction. Furthermore, a bulk method for manufacturing bulk PLZT includes a step of treating the PLZT at a high temperature of 1000° C. or higher after mixing a metal oxide constituting a raw material. Therefore, when such a method is applied to the element formation process, many restrictions are applied to the material selection, element structure, and the like.
For these reasons, instead of bulk PLZT, an approach involving the application of a thin film of PLZT formed on a base material to a light control element has been studied. Patent Reference 3 (Japanese Patent Application Kokai No. H7-146657) describes a display device in which a PLZT film is formed on a transparent substrate such as glass, and comb electrodes are provided on this film. This display device has a construction in which polarizing plates are provided on both surfaces of the display substrate on which the PLZT film is formed. Here, desired pixels are driven by connecting the electrode terminal parts of the respective pixels to an external driving circuit, so that a desired display can be performed by transmitted light from a light source provided on one side of the display substrate.
However, in order to put a light modulating film such as the PLZT film described above to practical use as an element such as a light shutter, it is necessary to create a driving circuit for controlling the on/off switching of a voltage that is applied to the light modulating film on a substrate, together with the light modulating film. In this case, in the construction described in Patent Reference 3, the region where the driving circuit is formed cannot be used as the display region, so that there is a problem in that an effective display region cannot be sufficiently obtained.
Furthermore, in the transmitting-type display device described above, when visible light is used as the illuminating light, the following problem is also encountered. More specifically, a driving circuit cannot be formed on a substrate including silicon or the like that is not transparent with respect to visible light. Moreover, in the display device described in Patent Reference 3, light loss caused by polarizing plates occurs because of the use of the polarizing plates.